[gdb/testsuite] Use -fuse-ld=gold in fission.exp

[gdb/testsuite] Use -fuse-ld=gold in fission.exp

The target board fission.exp requires the gold linker (because it supports
--gdb-index).

When running the target board on a system where the default linker is not
gold, most tests will fail to compile.

Fix this by adding "-fuse-ld=gold" ( supported in gcc since version 4.8).

gdb/testsuite/ChangeLog:

2019-06-18  Tom de Vries  <tdevries@suse.de>

	* boards/fission.exp (debug_flags): Add "-fuse-ld=gold".